The invention relates to a position detecting device for detecting the absolute value of a position of a movable member.
As conventional position detecting devices for detecting the position of a movable member, optical encoders such as linear or rotary encoders are known. The optical encoders have patterns, such as slits or diffraction grooves, formed on a scale by use of lithographic or other techniques. The encoders cause an optical sensor mounted on a movable member to irradiate the patterns with light, and detect the relative position of the movable member based on cyclic light reception signals according to the patterns. To detect the absolute position of the movable member, it is necessary to perform what is called the return-to-origin operation, i.e., to move the movable member to an initial position, on startup, in order for another sensor to detect the movable member at the initial position.
JP 2004-191267A discloses a position detecting device that includes a scale with absolute scale markings formed thereon. The absolute scale markings show respective pieces of information on absolute positions on the scale set at predetermined spacings therebetween, as pattern images such as bar codes. The conventional device eliminates the need for performing the return-to-origin operation by reading an absolute scale marking through an image sensor mounted on a movable member and detecting the absolute position of the movable member based on the information shown by the marking as read.
However, this device has the following problems. For high-precision detection of the position of the movable member, first, it is necessary to form on the scale a number of fine pattern images that show respective pieces of information on different absolute positions. Thus, the absolute scale markings need to be precisely formed by use of etching or other techniques, and it is, therefore, not possible to form a single long scale of a size that cannot be processed by the etching device. Different types of short scales, each with different absolute scale markings, are needed for being joined together for use as a long scale. This involves a complicated scale manufacturing process that results in a high production cost of the device.
Second, the fine pattern images as the absolute scale markings are more likely to be detected improperly under conditions such as of: the scale being dirty; vibrations; or there being a distance between the scale and the image sensor.
Finally, it is necessary to mount on the movable member the image sensor which includes a light emitting section, thereby causing a complex wiring or other internal structure for power feeding.
In view of the foregoing, a feature of the invention is to provide a position detecting device that detects precisely the absolute position of a movable member by a sensor, mounted on the movable member, for receiving light irradiated selectively from a plurality of light emitting elements mounted on a scale, while eliminating a complicated manufacturing process, nor a complex internal structure, that could cause an increase in manufacturing cost.